1. Field of the Invention
The present invention generally relates to a displacement pickup which picks up a displacement on the basis of an output from a vernier scale, and more particularly to a displacement pickup with a function to detecting an origin.
This application claims the priority of the Japanese Patent Application Nos. 2002-334246 filed on Nov. 18, 2002 and 2003-289354 filed on Aug. 7, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
The displacement pickup using a vernier scale is used in an X-ray pattern exposure system for manufacturing integrated circuits, precision machining, etc. In the displacement pickup of this type, a reference point or origin has to be set separately from an incremental signal indicating a travel of an object in order to measure an accurate position or distance.
Generally, in order to determine an origin through comparison of phases, first and second detected at a time, for example, of a vernier scale in the displacement pickup, the first and second phases can be compared (as in the Japanese Published Unexamined Patent Application No. 304112 of 1996). On the assumption that wavelengths of two signals supplied from the vernier scale are λ and (λ+λ/n), however, the two signals are coincident in phase with each other at intervals of λ(1+n) and a plurality of origins exist cyclically in the measuring direction (as in the Japanese Published Examined Patent Application No. 23618 of 1975), and the in-phase points where the signals are coincident in phase with each other are detected as a population having a certain width. Also, an interpolation is done to provide a resolution smaller than the wavelength of a reproduced incremental signal (as in the Japanese Published Unexamined Patent Application No. 122097 of 1996).
When the wavelength λ is sufficiently long, the plurality of origins can easily be discriminated between them, and an arbitrary one of the plurality of origins can be selected by an external fixed-point detecting means. If the wavelength λ is extremely short, the interval between the plurality of origins is small and difficult to select. For example, when wavelength λ is 0.137 μm and n=50, the interval will be 6.987 μm, so that an arbitrary one of the plurality of origins is difficult to select by the external fixed-point detecting means. It should be noted that n is an actual number other than zero.
To determine an origin through comparison between phases of the vernier scale used in the displacement pickup, two signals supplied from the vernier scale are coincident in phase with each other at intervals of λ(1+n) and a plurality of origins exist cyclically in the measuring direction. An arbitrary one has to be selected from the plurality of origins. However, it is very difficult to select, as a reference point, an arbitrary one of the origins from a population having a certain width, in which the in-phase points can be detected. Also in case a circuit error has taken place due to a power disconnection, detector replacement or the like, it is extremely difficult to always select the same origin. On the other hand, with the substantial increase in degree of recording density realized in the recent X-ray pattern exposure system or the like, the wavelength of the incremental signal has become shorter and the interval between the in-phase points has become smaller.